shmat/shmat01.c: cleanup

1. Remove useless comments

2. Revise code to follow ltp-code-style

Signed-off-by: DAN LI <li.dan@cn.fujitsu.com>
Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
shmat/shmat01.c: Test for specifying NULL to shmaddr

Test for statement:
"If shmaddr is NULL, the system chooses a suitable (unused) address
at which to attach the segment."

Signed-off-by: DAN LI <li.dan@cn.fujitsu.com>
Reviewed-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
mem.c: check whether MADV_MERGEABLE is available

Before using MADV_MERGEABLE, check whether it's available by macro
HAVE_MADV_MERGEABLE.

Signed-off-by: DAN LI <li.dan@cn.fujitsu.com>
Reviewed-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>